Determining Rumen Degradable Protein Requirements in Growing Beef Cattle

The objective was to assess requirements for RDP in growing beef cattle based on the 2016 Beef Cattle Nutrient Requirements Model. Six Simmental x Angus steers (BW = 324 ± 22 kg) with ruminal and duodenal cannulas were used in a replicated 3 × 3 Latin square design. Periods were 18 d with 13 d for adaptation and 5 d for total collection of feed, fecal, and urine samples. Blood urea N (BUN), ruminal pH, and ruminal NH₃ samples were collected on d 18. Steers were limit fed (2.3% of BW) diets with varying inclusions of urea formulated to be deficient in RDP (Low-RDP; CP = 13.2%; RDP = 5.5%), meet RDP requirements (Req-RDP; CP = 15.2%; RDP = 7.7%), or exceed RDP requirements (Hi-RDP; CP = 17.5%; RDP = 10.0%). While DMI was not affected (P = 0.20) by treatment, N intake increased (P < 0.01) with greater dietary RDP. Fecal N was not different (P = 0.69) across treatments, but steers fed the Hi-RDP diet had greater (P < 0.01) urine N compared with steers fed Req-RDP and Low-RDP diets. Steers fed Hi-RDP had greater (P = 0.01) apparent N digestibility than steers fed Low-RDP diets, with steers fed Req-RDP being intermediate and not different (P ≥ 0.10) from other treatments. Absorbed N was affected by treatment (P > 0.01) and increased with greater RDP. Retained N as a percentage of N absorbed was least (P ≤ 0.03) in Hi-RDP steers and did not differ (P = 0.99) between Req-RDP and Low-RDP steers. Retained N (P = 0.36), apparent dry matter digestibility (P = 0.4) and organic matter digestibility (P = 0.36) did not differ between treatments. A treatment × hour effect (P < 0.01) was observed for ruminal NH₃ as steers fed Hi-RDP diets had the greatest (P < 0.05) NH₃ 1, 2, 4, 8, and 12 h after feeding with steers fed Req-RDP being intermediate (P ≤ 0.05) at h 1 and 2. There was a treatment × hour effect (P < 0.01) for BUN as the greatest (P < 0.05) concentrations were observed in steers fed Hi-RDP at h 0 and h 4 with steers fed Req-RDP being intermediate (P ≤ 0.05) at h 4. Feeding steers Low-RDP resulted in decreased ruminal ammonia and BUN. Dietary RDP provided above requirements resulted in greater excretion of urine N that did not contribute to additional N retention.